Modification of chromatin structure via the reversible acetylation of nucleosomal histones plays an important role in the regulation of transcription in eukaryotic cells (reviewed in: Grunstein. 1997. Nature 389:349–352). Acetylation of the ε-amino group of specific lysine residues within the amino terminal tail of core histones results in localized chromatin relaxation. This acetylation is required to maintain the unfolded structure of nucleosomes undergoing transcription (Walia et al. 1998. J. Biol. Chem. 273:14516–14522). In general, histone acetylation activity correlates with transcriptional activation, whereas deacetylase activity correlates with transcriptional repression. Global changes in gene expression are thought to result from the dynamic equilibrium between histone acetyl transferase (HAT) enzyme activity and histone deacetylase (HDAC) enzyme activity within the nucleus of a cell.
HDACs mediate transcriptional repression by interacting with larger multisubunit complexes. For example, a specific HDAC known as HDAC1 is known to bind the corepressor Sin3, and HDAC1-Sin3 further associates with the silencing mediators NCoR and SMRT (Alland, et al. 1997. Nature 387:49–55). N-CoR/SMRT-HDAC1 is then recruited by specific transcription factors bound to promoter elements within the nucleus. For example, the retinoblastoma (Rb) gene product recruits N-CoR/SMRT-HDAC1 to bind the transcription factor E2F to repress E2F regulated promoters (Luo et al. 1998. Cell 92:463–473; Brehm et al. 1998. Nature. 391:597–600, Magnaghi-Jaulin et al. 1998. Nature 391:601–605). HDAC1-Sin3 also binds to and mediates repression by the MAD/MAX repressor heterodimer (Laherty et al. 1997. Cell 89:349–356). The histone deacetylation activity of HDAC1 is essential for this transcriptional repression (Hassig et al. 1998. PNAS 95:3519–3524).
Several cDNAs encoding histone deacetylases have been characterized. The first to be cloned was the yeast protein RPD3, which was first identified in genetic screens for transcriptional repressors (Vidal et al. 1991. Mol. Cell Biol. 11:6317–6327). Mammalian HDAC1 was cloned independently as the molecular target of TSA (Taunton et al. 1996. Science 272:408–411). HDAC1 was observed to be an ortholog of yeast RPD3, and both were shown to have HDAC activity in vitro. Three cDNAs, HDAC1, HDAC2, and HDAC3, encoding proteins homologous to RPD3 have been described (Yang et al. 1997. J. Biol. Chem. 272:28001–28007; Emiliani et al. 1998. PNAS 95:2795–2800). All three have ubiquitous tissue distributions, and their activities appear to be overlapping. To date, five S. cerevisiae genes have been found which encode HDAC's, and the family has been divided into two classes based on sequence conservation and proposed biochemical function. S. cerevisiae Hos1, Hos2, Hos3, and RPD3, together with mammalian HDAC1, 2, and 3 comprise class 1. The second class is comprised of the yeast HDA1 and HDA3 and the recently identified human homologs HDAC4, HDAC5, and HDAC6 (Grozinger et al. 1999. PNAS 96:4868–4873; Fischle et al. 1999. J. Biol. Chem. 274:11713–11720).
As HDACs are known to function as part of multisubunit transcriptional regulatory complexes, regulating HDACs may be used to modulate, i.e., increase or decrease, the transcription of a specific gene or set of specific genes under the control of a regulatory complex containing an HDAC (Carmen et al. 1996. J. Biol. Chem. 271(26):15837–15844.
A variety of compounds possessing HDAC inhibitory activity have been shown to lead to hyperacetylation of histones, and this modification is accompanied by cell cycle arrest and terminal differentiation in a number of cell types (See Proceedings of the American Association for Cancer Research 40, March, 1999). Like NaBu, the fungal toxin trichostatin A (TSA), a potent, specific, and reversible inhibitor, has been shown to induce cell cycle arrest and/or differentiation in a variety of systems. Recently, a synthetic benzamide derivative (MS-27275) with HDAC inhibitory activity was described (Saito et al. 1999. PNAS 96:4592–5497). MS-27275 induces histone hyperacetylation, and demonstrates efficacy in inhibiting tumor growth in tumor bearing nude mice.
In acute promyelocytic leukemia (APL), a genetic rearrangement results in the fusion of the retinoic acid receptor α (RARα) with either the PML or PLZF proteins. Both PML-RARα and PLZF-RARα aberrantly recruit N-CoR/SMRT-HDAC and mediate leukaemogenesis by repressing retinoic acid regulated genes (Lin et al. 1998. Nature 391:811–814; Grignani et al. 1998. Nature 391:815–818). Indeed, the therapeutic use of sodium butyrate (NaBu), a nonspecific HDAC inhibitor, to target transcription in APL has been successful even in retinoic acid resistant forms of the disease (Warrell et al. 1998. J. Natl. Cancer Inst. 90:1621–1625). A role for HDAC has also been demonstrated in acute myeloid leukemias that arise as a result of the fusion of the AML1 transcription factor with the ETO (“eight twenty one” or MT68) gene product (Gelmetti et al. 1998. Mol. Cell Biol. 18:7185–7191; Wang et al. 1998. PNAS 95:10860–10865). Together, this suggests that inhibition of HDAC is a viable approach to the treatment of certain leukemias. There is further evidence for a role of HDAC in other types of cancers, including colon cancer (Hassig et al. 1997. Chem. Biol. 4:783–789; Archer et al. 1998. PNAS 95:6791–6796), squamous cell carcinoma (Gillenwater et al., 1998. Int. J. Cancer 75:217–224; Saunders et al. 1999. Cancer Res. 59:399-404), adenocarcinomas (McBain et al. 1997. Biochem. Pharmacol. 53:1357–1368), and neuroblastomas (Swendeman et al. 1999. Proc. Amer. Assoc. Cancer Res. 40, Abst. #3836). HDACs have also been implicated in transcriptional regulation mediated by BRCA1 (Yarden et al. 1999. Proc. Ameri. Assoc. Cancer Res. 40, Abstr. #3387) and c-fos (Bakin and Curran. 1999. Science 283:387–390).
On the other hand, it is also sometimes desirable to enhance proliferation of cells in a controlled manner. For example, proliferation of cells is useful in wound healing and where growth of tissue is desirable. Thus, identifying modulators which promote, enhance or deter the inhibition of proliferation is desirable.
Despite the desirability of identifying transcription components and modulators, there is a deficit in the field of such compounds. Accordingly, it would be advantageous to provide compositions and methods useful in screening for modulators of transcription. It would also be advantageous to provide novel compositions which are involved transcription regulation.